Device for testing the adequacy of electrical circuit such as the ground circuit of an extension cord

ABSTRACT

Device for testing the adequacy of the ground and power circuits of electric tools, extension cord and the like by performing an automated series of individual tests in sequence. The major tests are to determine: open ground circuit, faulty ground circuit, short circuit and poor insulation. If no faults are found by these tests, the circuit under test is next connected to line potential to determine if it operates properly. During this phase of testing, the potentials and currents associated with the previous tests are discontinued to avoid power drain and heating. A saturating current transformer protects an essential relay from damage from excessive current. A long extension cord having a proper ground resistance (in the order of the resistance exhibited by a tool having a faulty ground cord) may be tested by modifying the test currents so that an erroneous faulty ground indication is not given.

United States Patent 1191 Ettelman 1451 Aug. 14, 1973 DEVICE FOR TESTINGTHE ADEQUACY OF ELECTRICAL CIRCUIT SUCH AS THE GROUND CIRCUIT OF ANEXTENSION 211 Appl. 1%.; 164,072

Related US. Application Data [62] Division of Ser. No. 857,796, Sept.15, 1969, Pat. No.

52 us. c|....., 324/51, 324/73 51 1111. C1...- G0lr 31/02, G01r 15/12 58Field of Search "324/51, 73; 340/255;

[56] References Cited UNITED STATES PATENTS 2,858,507 10/1958 Liautaudet aL... 324/73 R 3,402,326 9/1968 Guasco et a1.. 317/18 A 3,515,9416/1970 Moore et a1. 317/18 A VOLTAG E S ECO N DAEY HlGH VOLTAGESECONDARY LOW VOLTAGE S ECONDARV SA E OPERATING CONDITION 1 1 I LAMPOPERATE TEST TEST 3,611,133 10/1971 Tlergal ..324/51X PrimaryExaminerGerard R. Strecker Att0rneyRobert Scobey et a1.

[5 7] ABSTRACT Device for testing the adequacy of the ground and powercircuits of electric tools, extension cord and the like by performing anautomated series of individual tests in sequence. The major tests are todetermine: open ground circuit, faulty ground circuit, short circuit andpoor insulation. If no faults are found by these tests, the circuitunder test is next connected to line potential to determine if itoperates properly. During this phase of testing, the potentials andcurrents associated with the previous tests are discontinued to avoidpower drain and heating. A saturating current transformer protects anessential relay from damage from excessive current. A long extensioncord having a proper ground resistance (in the order of the resistanceexhibited by a tool having a faulty groundcord) may be tested bymodifying the test currents so that an erroneous faulty groundindication is not given.

4 Claims, 5 Drawing Figures PAIENIEMHMH 3.753.088

sm 1 or 3 Tic]. 1.

HIGH VOLTAGE SECON DARY LOW VOLTAGE L sscouomv i I 45w, I F 43 m SAFEOPERATING COMDIFIOM l 2 a2 2 I 1 4 I sc t i i 5 I I I I 66 m f' I III I3 l PRIMARY 70 I FAUL Y I 1 I I I I 4h GROUND g f 3 2H :cuzcun M i a 78* POWER 1 g I .ififi I L3 LAMP OPERATE Tia. E.

Pmmmu: 14 an 3. 753 088 W) ma I B i 1 DEVICE FOR TESTING THE ADEQUACY OFELECTRICAL CIRCUIT SUCH AS THE GROUND CIRCUIT OF AN EXTENSION CORD CROSSREFERENCE TO RELATED APPLICATION BACKGROUND AND SUMMARY OF THE INVENTIONThe invention is in the field of devices for testing the adequacy ofelectric tools, such as electric drills, electric motors, solderingirons, and the like, and for testing the adequacy of electricconductors, such as extension cords and the like. It is an improvementof the testing device disclosed in US. Pat. No. 2,858,507, issued Oct.28, 1958 to Liautaud et a].

For various reasons electric tools and extension cords develop a varietyof electrical defects which may be dangerous to an operator, or mayotherwise impede the usefulness of the tool or extension cord. In orderto prevent danger to personnel, and in order to prevent work delays dueto lack of properly operable equipment, electric tools and extensioncords may be inspected periodically for actual and potential defects. Tocarry out such inspections by skilled personnel may, however, becomeexcessively expensive and time consuming, particularly in a largeindustrial installation where a great number of tools and extensioncords may be in use.

There is a need, therefore, for acompact testing device which may beused by unskilled personnel and which would indicate quickly and in aneasy-tounderstand fashion whether a tool or an extension cord hasdeveloped or is developing electrical defects, and the nature of suchdefects.

There have been various testing devices used for this purpose in thepast, one of which is described in the US. patent to Liautaud et al.,above. The improvement to which the present application is directedincludes making special provisions for testing long extension cords,particularly, to modify the test currents so that an extension cordhaving a proper ground resistance which is in the order of a faultyground resistance for the cord of a tool does not result in a faultyground indication. Broadly stated, the present invention is directed toa device for testing different circuits in which electricalcharacteristics (e.g., resistances) may vary, but which includes meansfor varying the indication otherwise produced by an indicating device sothat substantially identical output indications are provided underappropriate circumstances when the different circuits are tested.

The tests described below are designed to detect the existence of and toidentify various electrical defects in equipment. In the course of itsoperation, the testing device may subject a tool or a cord to voltagesand currents which exceed the normal voltages and currents which thetool or cord encounters in normal use, for the purpose of detectingcertain defects which may not be immediately apparent in normaloperation.

Lil

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary perspectiveview of a testing device in accordance with the invention.

FIG. 2 is a schematic diagram of circuitry embodying the invention.

FIG. 3 is an abbreviated schematic diagram of a part of the circuit ofFIG. 2 which is used to provide overload protection by means of asaturating current transformer.

FIG. 4 is an abbreviated schematic diagram of a part of the circuit ofFIG. 2 used during testing the adequacy of the ground circuit of anextension cord.

FIG. 5 is a schematic circuit diagram of a circuit similar to that ofFIG. 3 to permit ground circuit testing involving relatively lowmagnitude currents.

DETAILED DESCRIPTION The testing device may be housed in a structuresuch as a box 104 (conductive or nonconductive), illustratedschematically in FIG. 1. A pair of conductive (metal) brackets 106,which may be provided for sup porting an electric tool under test,extend from the box 104 and are connected electrically to the boxground.

The exemplary tool illustrated in FIG. 1 is a standard electrid drillwhich has a metal housing 108 making electrical contact with thesupporting brackets 106. The drill has a cord 102 which, as seen in theexploded view thereof, includes three leads, each having a sleeve ofinsulating material enclosing a conductor. The reference numerals 102-a,l02-b, and 102-0 refer respectively to the two power leads and to theground lead which is connected to the housing 108. The cord 102terminates in a male plug 109 which, depending on the configuration ofits prongs, may be plugged into one of receptacles 80, 81 and 82. Shouldthe male plug 109 be incompatible with any of the shown receptacles, anappropriate adaptor may be used.

Also shown in FIG. 1 is an extension cord which may terminate at one endof a male plug 110 for plugging, directly or through an adaptor, intoone of the receptacles 80, 81 and 82, and may terminate at the other endin a female receptacle 11] for connection, through an appropriateadaptor 112, to a receptacle 83. The leads of the extension cord 100 areshown in an ex ploded view where the reference numerals 100-a, 100-b and100-c refer respectively to the two power leads and to the ground lead.Each of the leads is enclosed in a sleeve of insulating material brokenoff to show the conducting lead. i

The tool and extension cord have been shown together in FIG. 1 forconvenience. In use only one device is tested at any time.

Reference numeral 84 refers to a jack connected to the ground prongs ofthe receptaclesfifl, 81 and 82, and reference numeral 85 refers to ajack connected to the ground of the box 1 For the purpose of allowingthe tester to follow its preset sequenceof steps in testing a two-wiretool or extension cord (noground lead). jacks 84 and 85 are connected toeach other by means of a jumper lead (not shown). In the above case, theterm ground circuit" of a tool or an extension cord includes the jumperleads associated with the jacks 84 and 85. The term "power circuit"includes thecircuit between the prongs of the power leadsofacordassociated with a tool or an extension cord.

FIG. 2 illustrates the electrical circuitry housed in the box 104. Thecircuit employs relays CR-l through CR-S, respectively having windings10, 20, 30-a and 30-b, 40 and 50, and a number of contacts (shown in thedeenergized states of the windings) along the broken lines runningvertically in the figure and intercepting the windings. A Timerrepresented by a winding 120 and contacts 121 and 122 starts runningwhen the winding 120 is energized and runs for a predetermined timeinterval at the end of which it closes contacts 121 and opens contacts122. The circuit also includes indicator lights 61 through 67 and abuzzer 68, each of which is turned on by current flow through it.

A power transformer having a primary winding 70 and secondary windings71, 72 and 73 is included. The circuit of FIG. 2 is energized through areceptacle 86 connected to a source of AC. potential through an Off- Onswitch 90 and through an appropriate fuse 87. A single throw-six poleswitch S-2 labeled Lamp Test has normal open contacts 1 through 6 and isused to turn on the indicator lights 61, 62, 63, 64 and 66 and thebuzzer 68, so that an operator of the testing device can make sure thatthe lights and the buzzer are in good order before proceeding with thetest. A single throwthree pole switch S-3 labeled Operate Test hascontacts 7, 8 and 9, and is used to initiate the testing procedure ofthe testing device.

An electric device, such as an electric drill, is positioned on thebrackets 106 and connected as previously described.

Once the tool to be tested is properly connected, the Off-On switch 90is closed and a lamp and buzzer test may be completed by the operator ofthe testing device by momentarily closing the Lamp Test switch S2.

- When the switch S-2 is closed, current flows through its contacts 1through 6, and indicator lamps 61, 62, 63, 64 and 66, as well as thebuzzer 68 should be activated. If one or more of the indicator lightsfail to light, or if the buzzer 68 fails to sound, the testing deviceshould be repaired before the test is proceeded with. Once it isestablished that the testing device is in proper order, the testprocedure may be started by closing the Operate Test switch S-3 andholding it in closed position so that current can flow through itscontacts 7, 8 and 9. Upon the closing of the contacts 7, current flowsfrom the power line lead 90-a through the closed contacts 7, the primarywinding 70 of the power transformer, the normally closed contacts 11 ofthe relay CR-l, to the power lead 90-h. All power transformer secondarywindings are now energized.

A part of the circuit associated with the secondary winding 73 isillustrated schematically in FIG. 3. The secondary winding 73 of thepower transformer is in series with the primary winding of a saturatingcurrent transformer 74. The secondary winding of the transformer 74 isin series with the winding 50 of the relay CR-S. The primary winding ofthe transformer 74 is connected in a circuit including winding 30-a ofthe relay CR-3, a variable resistor 92, the ground lead 102-0 of thecord 102 connected to the ground prong of the receptacle 80-c (forexample) and the metal housing 108 of the tool under test connected tothe brackets 106. The purpose of the transformer 74 is to protect thewinding 50 from excessive amounts of current for long time periods thatflow during a ground conductor test. Here, the heavy current from thepower transformer secondary 73, which circulates in the primary oftransformer 74, appears as a lesser current in the secondary circuit.Thus the current through winding 50 is limited to the value determinedby the saturating characteristics of the transformer 74.

The circuit of FIG. 3 otherwise operates in the same fashion as in theLiautaud et al. patent mentioned above, except that variable resistor 92permits a sensitivity adjustment normally required only at assembly ofthe testing device. For example, to explain the operation, a groundcircuit resistance of over about 2 ohms, representing an open groundcircuit, causes winding 30-a to be energized to a point which actuatesrelay CR-3; a resistance of approximately 1 ohm, representing a faultyground circuit, causes both windings 50 and 30-a to be energized topoints which actuate concurrently relays CR-S and CR-3, respectively; aresistance of approximately one-half ohm or less, representing a goodground circuit, causes winding 50 to be energized to a point whichactuates relay CR-S. In all of these cases, the appropriate indicatorlight is energized. In the first two cases, the buzzer 68 also soundsand the timer winding 120 is not energized thereby preventing completionof the cycle. In the last case, since it represents a safe operatingcondition, the timer winding '120 is energized and the cycle isautomatically completed.

The saturating current transformer 74 operates in the same fashion tolimit current flow through winding 50 in the testing of extension cords.Electric cords are subjected to a set of tests similar to that appliedto electric tools (circuits of a first type), but not identical, becauseextension cords (circuits of a second type) have certain uniquecharacteristics (resistances). For example, while the ground circuit ofelectric tools is generally short usually 6 feet and almost always lessthan 25 feet, the ground circuit of an extension cord may be anywherefrom a few to over a hundred feet. If the ground circuit of a goodextension cord is, for example 25 feet, its resistance is less thanone-half ohm and it can be successfully tested in a manner identical tothat applied to ground circuits of electric tools. But if the groundcircuit of an extension cord is of the order of, say, 100 feet, itsresistance may be nearer 1 ohm and a test identical to that applied toelectric tools may indicate a faulty ground circuit, even though theextension cord may be in perfect shape. The test device decribed belowobviates problems of this nature and gives indications of the adequacyof extension cords of a wide range in length.

Once the extension cord is connected properly, the testing device isturned on and switches S-2 and S-3 are operated, all as previouslydescribed. Current flow is now established through the primary winding70 of the power transformer and all secondary windings are energized (togenerate testing electrical signals) as described above.

FIG. 4 shows the part of the circuit of FIG. 2 under consideration.Saturating current transformer 74 operates to limit current flow inwinding 50 as described above. Variable resistor 92 and winding 30a ofthe relay CR-3 are included in the circuit as in FIG. 3. New to thecircuit for extension cord testing (i.e., active only during a test of asecond type" of circuit) is a winding 30-12 in series with groundconductor -0. The winding 30-b is wound together with the winding 30-aof the relay CR-3 such that the electromagnetic fields of the twowindings interact. The two windings are connected so that the fieldstherefrom oppose each other (winding 30-b is a bucking coil). As apractical example. winding 30-a may include ten or more times tlieturnsof winding 30-h and be of a wire diameter much smaller than that ofwinding 30-12. The result is that a current such as one-half ampereflowing in winding 30-a will operate relay CR-3 with no current in 30-12as in the case of an open ground. On the other hand, with the sameonehalf ampere flowing in 30-a, currents in the range of -20 amperes in30-b (e.g., good ground in a long extension cord) will prevent operationof CR-3 (varying the out-put indication otherwise provided) and therebyprevent a misleading indication of a faulty ground. It should be notedthat the variable resistor 92 permits adjustment of the current flowthrough winding 30-41.

ELECTRIC CORD OPEN GROUND CIRCUIT As a first situation, suppose that theground lead 100-c is open, e.g., that its resistance is more than 2ohms. Most of the voltage generated by the secondary winding 73 (seeFIG. 4) appears across the points A and B because the overall currentfrom winding 73 is relatively low. Under these conditions winding 30-acontrols, energizing relay CR-3. In reference to FIG. 2, once the relayCR-3 is energized, its normally closed contacts 32 and 34 open, and itsnormally open contacts 31 and 33 close. Upon the closing of contacts 33,the buzzer 68 is turned on by the circuit established through contacts42 and 33. The Open Ground indicator light 64 is turned on in thecircuit established by the closed contacts 8, 52 and 1 1. The SafeOperating Condition light 66 is not energized because the contacts 32,in series with it, are open. Thus, the buzzer 68 is on to indicate thepresence of a defect, and the Open Ground indicator light 64 is on toindicate the nature of the defect. Although the contacts 31 are closed,the Faulty Ground light 61 is not energized because relay CR-S is notactuated and contacts 51 are open. CR-S is not actuated since thecurrent through winding 50 is insufficient. The testing device cannotproceed with further tests because contacts 54 of relay CR-5 remainopen, preventing winding 120 of the Timer from being energized. The testmay be discontinued by opening the Off-On switch 90 and the cord undertest may be unplugged.

ELECTRIC CORD FAULTY GROUND CIRCUIT As a second situation in this set oftests, suppose that the ground circuit of the extension cord has aresistance slightly over 1 ohm, for example. The testing de viceoperates in the same manner as with an electric tool whose groundcircuit is between land 2 ohms in resistance, i.e., both current relaysCR6 and ca s are energized, and the Faulty Ground indicator light 61 andthe buzzer 68 are turned on. The relay CR-5 is energized because anappreciable current flows through winding 74. Current flow throughbucking coil 30-b is not sufficient to prevent energization of relayCIR-3 by winding 30-a, which still controls because an appreciablevoltage still appears across points A and B. The buzzer 68 indicatesthat the cord under test is defective and the Faulty Ground indicatorlight 61 shows the nature of the defect. The Open Ground light 64 is notenergized because contacts 52 are open due to actuation of relay CR-S.

After a Faulty Ground circuit has been detected, the Timer circuit isprevented from being energized as described above.

ELECTRlC CORD GOOD GROUND CIRCUIT But, suppose as a third situtation,that the resistance of the ground circuit under test is considerablyless than in the two previous cases, e.g., below 1 ohm. The

effect of the voltage appearing across the winding 30-a and the currentthrough winding 30-11 is such that the fields from the two windingscancel each other, preventing actuation of relay CR-3. The net currentthrough winding 74 is sufficient to cause actuation of relay CR-S.

Upon the actuation of relay CR-5, other tests proceed, as set forth inthe Liautaud et al. patent noted above, involving high voltage secondary72 and low voltage secondary 71. The high voltage of secondary 72 isapplied between the power conductors of the extension cord and ground,and any leakage therebetween sufficient to energize winding 40 to apoint sufficient to actuate relay CR-4 results in Power Ground light 63being turned on. The low voltage of secondary 71 is applied between thepower conduct-ors of the extension cord. Any unusually low resistancebetween these two conductors results in a current sufficient to energizewinding 20 to a point sufficient to actuate relay CR-2 and turn on ShortCircuit light 62. Realys CR-2 and CR-4 are adjusted to sensitivitiescommensurate with the currents to be measured.

At this same time, and as long as relays CR-2 and CR-4 are not actuated,the Safe Operating Condition light 66 is on in the circuit establishedthrough contacts 42, 32, 22, 9 and 122 to indicate that the groundcircuit of the cord under test is adequate. The Open Ground indicatorlight 64 is turned off because of the opening of contacts 52 in serieswith it. The Timer motor is now turned on because its winding 120 is ina circuit established through contacts 42, 32, 22, 54, and contacts 11.The Timer starts running through its predetermined time delay.

Once the delay period is over, the normally open contacts 1121 close thenormally closed contacts 122 open. With the closing of contacts 121, acircuit including the winding 10 of the relay CR! is established, andrelay CR4. is actuated. Contacts 11 of relay (JR-ll now open, and thepower transformer primary 70 no longer has its rated voltage applied toit. All secondary wind ings '71, 72 and 73 are deenergiaed and no heatgenerating currents can flow therefrom. In particular, the low voltagefrom secondary winding 73 is removed by deencrgization of the winding,and the heat generation by the current flow therefrom is discontinued.In the Liautaud et al. patent noted above, the secondary windings 71 and72 are decoupled from the circuit under test. The secondary winding 73,however, remains coupled to the circuit under test, and the substantialcurrent flowing therefrom results in substantial heat generation. It ispossible to decouple the secondary winding 73 in the same fashion as arethe other two secondary windings. However, arcing across any contactsused to decouple is one reason why deenergization of the secondarywinding 73 is preferred. By virwe of the actuation of relay CR-l,contacts 13 and 15 apply line voltage to the extension cord under test.Light 67 will turn on, indicating continuity, if the extension cord iscontinuous. If this light 67 does not turn on, a break in the cord isindicated.

The action of the Timer just explained to prevent heat generation is thesame for tools as well as cords.

It should be noted in connection with FIG. 2, that the resistor 93serves to limit current, while resistor 47 is a sensitivity adjustmentfor the winding 40 of relay CR4. The diodes 45 and 46 serve to provideD.C. operation and are suppression, respectively, if desired inconnection with winding 40.

FIG. shows a circuit similar to FIG. 3 for testing equipment in whichthe testing currents applied to the ground circuit under test must belimited in magnitude to a value much lower than encountered in thecircuit in FIG. 3, e.g., in the neighborhood of 20 percent of that ofthe FIG. 3 circuit. The circuit of FIG. 5 uses the winding 30-1) of FIG.4 for this purpose. Like reference numerals have been used in FIG. 5 todesignate like components with respect to FIG. 3. In FIG. 5 winding 50'is similar to winding 50 of FIG. 3; a saturating current transformer isnot required in the circuit of FIG. 5, since the currents are much lowerand there is no danger of overloading the winding 50. Winding 30-12 isthe same as that in FIG. 4, except it is connected differently in thecircuit so that the fields from windings 30-0 and 30-b aid each other.Windings 30-a and 30-b actuate the relay CR-3. The variable resistor 92'is similar to variable resistor 92 and is used to adjust the currentflowing through winding 30-h, normally only at assembly of the testingdevice.

In the circuit of FIG. 5, the same resistances in ground circuit 100-0of equipment 108 under test as in the circuit of FIG. 3 are used as anindication of ground circuit quality, e.g.:

open ground circuit greater than 2 ohms;

faulty ground circuit approximately 1 ohm;

good ground circuit less than one-half ohm. Since the testing currentsare restricted to much lesser values, the winding 30-h is used toprovide a field which aids that from winding 30-a to properly actuatethe relay CR-3.

I claim:

1. A device for testing the adequacy of electric circuits of both afirst and a second type, in which an electrical characteristic of acircuit of the first type is adequate if within a first range and theelectrical characteristic of a circuit of the second type is adequate ifwithin a second range different from said first range, including meansfor generating testing electrical signals, means for applying saidsignals to a circuit under test which is one of said two type, and meansfor detecting the electrical response of the circuit under test andproviding an indication of said resonse which is representative of saidelectrical characteristic of the circuit under test, wherein theimprovement comprises:

means coupled to said detecting means and active only when a circuit ofone of said types is under test for varying the indication provided bythe detecting means to produce substantially identical indications whena first type circuit under test has its characteristic within said firstrange and a second type circuit has its characteristic within saidsecond range.

2. A device for testing the adequacy of electrical circuiJs of more thanone type of electrical device, such as the ground circuit of anextension cord and the ground circuit of an electrical tool, includingmeans for generating testing electrical signals coupled to a parallelcombination having as one leg an electrical circuit under test and asthe other leg a first winding, the field from said winding controllingthe energization of an indicating device, wherein the improvementcomprises:

a second winding active only when an electrical circuit of oneparticular type is under test and responsive to the current flowing in aportion of the combination for providing a field coacting with the fieldfrom the first winding to vary the energization of said indicatingdevice to cause substantially identical output indications by saidindicating device for difierent types of circuits under testnotwithstanding the fact that the currents through said first windingvary for said different types of circuits under test.

3. A device as in claim 2, in which the second winding is in the legwith the circuit under test and produces a field opposing the field fromthe first winding.

4. A device as in claim 2, in which the second winding is responsive tosaid testing electrical signals and produces a field that adds to thefield from the first winding.

i i i i

1. A device for testing the adequacy of electric circuits of both afirst and a second type, in which an electrical characteristic of acircuit of the first type is adequate if within a first range and theelectrical characteristic of a circuit of the second type is adequate ifwithin a second range different from said first range, including meansfor generating testing electrical signals, means for applying saidsignals to a circuit under test which is one of said two type, and meansfor detecting the electrical response of the circuit under test andproviding an indication of said resonse which is representative of saidelectrical characteristic of the circuit under test, wherein theimprovement comprises: means coupled to said detecting means and activeonly when a circuit of one of said types is under test for varying theindication provided by the detecting means to produce substantiallyidentical indications when a first type circuit under test has itscharacteristic within said first range and a second type circuit has itscharacteristic within said second range.
 2. A device for testing theadequacy of electrical circuiJs of more than one type of electricaldevice, such as the ground circuit of an extension cord and the groundcircuit of an electrical tool, including means for generating testingelectrical signals coupled to a parallel combination having as one legan electrical circuit under test and as the other leg a first winding,the field from said winding controlling the energization of anindicating device, wherein the improvement comprises: a second windingactive only when an electrical circuit of one particular type is undertest and responsive to the current flowing in a portion of thecombination for providing a field coacting with the field from the firstwinding to vary the energization of said indicating device to causesubstantially identical output indications by said indicating device fordifferent types of circuits under test notwithstanding the fact that thecurrents through said first winding vary for said different types ofcircuits under test.
 3. A device as in claim 2, in which the secondwinding is in the leg with the circuit under test and produces a fieldopposing the field from the first winding.
 4. A device as in claim 2, inwhich the second winding is responsive to said testing electricalsignals and produces a field that adds to the field from the firstwinding.